Guided wave optical switches/modulator demonstrated to date operate effectively for only a single direction of wave polarization. However, available single mode fibers do not preserve any particular direction of polarization. As a result, a linearly polarized optical signal applied at the input end of a fiber emerges at the output end with an arbitrary elliptical polarization that can vary with time. Under these circumstances, a single polarization switch would yield unacceptably high crosstalk and loss whenever the polarization of the received signal is different than the particular polarization for which the switch is designed.
Efforts to obviate this problem have been directed to both the fiber and the coupler. With respect to the former, specially fabricated birefringent fibers, that maintain linear polarization, are currently under investigation and have been demonstrated for only short lengths. In addition, questions concerning loss, cabling and splicing have not been addressed.
With respect to the switch/modulators, the difficulty of achieving efficient directional coupler switching (i.e., low channel crosstalk) for both the TE and the TM modes by means of a common applied voltage resides in the fact that the orthogonal modes see different electrooptic coefficients. As a result, the induced phase mismatch, .DELTA..beta., produced by a common applied voltage, is different for the two polarizations. In addition, because the guide-substrate refractive index difference, .DELTA.n, is generally different for the TE and TM modes, the mode confinement and, consequently, the interguide coupling strength, k, is polarization sensitive. Inasmuch as k, .DELTA..beta. and the interaction length, L, determine the crossover efficiency, the polarization sensitivity of the switch state is readily apparent.
Efforts to minimize these difficulties have involved the use of separate applied voltages for controlling the two polarizations independently. (See, for example, "Polarization-Insensitive Integrated-Optical Switches: A New Electrode Design" by R. A. Steinberg et al., Applied Optics, Vol. 16, No. 8, August, 1977. Also, U.S. Pat. No. 4,157,860 issued to E. A. J. Marcatili.) In general, the prior art technique is to provide two separate couplers in a single structure (i.e., one for each mode), the assumption being that each of the orthogonally applied voltages will affect only one of the two polarizations. In practice, however, second order effects can be significant, requiring a time consuming iterative tuning procedure to achieve good channel isolation for both polarizations.